Presented byDr Andrew HodgesMathematician and author

Who was Alan Turing?

Alan Turing was not a well known figure during his lifetime. But today he is famous for being an eccentric yet passionate British mathematician, who conceived modern computing and played a crucial part in the Allied victory over Nazi Germany in WW2.

He was also a victim of mid-20th Century attitudes to homosexuality – he was chemically castrated before dying at the age of 41.


Discouraged at school

Sherborne School Archives

Sherborne School Archives

Alan Turing, aged 15, at Westcott House, Sherborne School.

Alan Turing spent much of his early life separated from his parents, as his father worked in the British administration of India.

At 13 years old, he was sent to Sherborne School, a large boarding school in Dorset. The rigid education system gave his free-ranging scientific mind little encouragement, so Turing studied advanced modern scientific ideas, such as relativity, on his own, running far ahead of the school syllabus.

See excerpts from Turing's school reports

If he is to be solely a Scientific Specialist, he is wasting his time at a Public School.

Alan Turing's headmaster, Sherborne School


Devastated but inspired by his friend's death

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Andrew Hodges talks about the profound effect Christopher Morcom’s death had on Alan Turing. Clip from Horizon (BBC Two).

Transcript (PDF 117k)

The situation changed when Alan Turing became intensely attracted to another able pupil, Christopher Morcom.

He was inspired to communicate more and also to become an academic success. But Christopher died suddenly from tuberculosis. Devastated, Turing wanted to believe that Christopher’s mind somehow lived on. His emotional turmoil involved a scientific fascination with the problem of mind and brain that underlay his later work.

Discover more about Alan and Christopher's friendship


A new home in Cambridge

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getty Images

Turing’s work in probability theory won him a Fellowship of King's College, University of Cambridge, in 1935.

Turing won a scholarship to King's College, Cambridge, and took the Mathematics degree with distinction.

He thrived in a culture that encouraged his scientific interests and as a young gay man he also found protection in the liberal ambiance the college provided. At just 22, he was elected to a Fellowship. Turing was already on track for a distinguished career in pure mathematics. Yet his unusual interest in finding practical uses for abstract mathematical ideas was to push him in an altogether different direction.

Find out more about Turing's life at King's College


Founder of modern computing

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Jim Al-Khalili explains how Turing invented the idea of feeding one machine different instructions. Clip from Order and Disorder (BBC Four).

Transcript (PDF 63k)

In 1936, Turing published a paper that is now recognised as the foundation of computer science.

Turing analysed what it meant for a human to follow a definite method or procedure to perform a task. For this purpose, he invented the idea of a ‘Universal Machine’ that could decode and perform any set of instructions. Ten years later he would turn this revolutionary idea into a practical plan for an electronic computer, capable of running any program.

Simple demonstration: How does a Turing machine work?Turing's original paper


Breaking the Enigma code

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The Bombe – designed by Turing – was used to crack Nazi ciphers. Marcus du Sautoy watches it in action. Clip from Horizon (BBC Two).

Transcript (PDF 63k)

After two years at Princeton, developing ideas about secret ciphers, Turing returned to Britain and joined the government’s code-breaking department.

In July 1939, the Polish Cipher Bureau passed on crucial information about the Enigma machine, which was used by the Germans to encipher all its military and naval signals. After September 1939, joined by other mathematicians at Bletchley Park, Turing rapidly developed a new machine (the ‘Bombe’) capable of breaking Enigma messages on an industrial scale.

More on the Enigma machineHow did code breakers change the course of the war?What else did Turing work on at Bletchley Park?


End of a brief engagement

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Joan Murray, formerly Joan Clarke, talks about her relationship with Alan Turing. Clip from Horizon (BBC Two).

Transcript (PDF 64k)

In 1941, Turing’s section, ‘Hut 8’, mastered the German submarine communication system that was vital to the battle of the Atlantic.

In the course of this exciting work he found the friendship of another mathematician, Joan Clarke. Turing proposed to her, but immediately told her of his ‘homosexual tendencies’, and the engagement soon ended. After this, he became more confident in developing his homosexual life. Meanwhile, the war took a new turn as America joined the war.

Discover more about Joan Clarke's remarkable career


The electronic connection

Jack Copeland

Jack Copeland

Turing was recruited to the National Physical Laboratory in 1945.

Turing worked on other technical innovations during the war – in particular, a system to encrypt and decrypt spoken telephone conversations.

Codenamed Delilah, it was successfully demonstrated using a recording of one of Winston Churchill's speeches, but was never used in action. However, it gave Turing hands-on experience of working with electronics, and led to a position at the National Physical Laboratory (NPL), where he worked on what he sometimes described as an ‘electronic brain’.


Designs a first electronic computer

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Engineer Donald Bayley describes Turing’s frustration at not being able to work on the construction of the ACE. Clip from Horizon (BBC Two).

Transcript (PDF 65k)

In March 1946 Turing produced a detailed design for what was called the Automatic Computing Engine (ACE.)

This was a digital computer in the modern sense, storing programs in its memory. His report emphasised the unlimited range of applications opened up by this technological revolution, and software developments ahead of parallel American developments. Yet his relationship with NPL soured and he left in 1948, before a pilot version of the ACE was made in 1950.

Can Alan Turing be called the 'Father of Computing'?


Can a machine think?

The University of Manchester

The University of Manchester

A team of scientists work on the Baby – formally known as the Manchester Small-Scale Experimental Machine.

Turing moved to the University of Manchester, where electronic engineers had already demonstrated a very small stored-program computer.

Now he focused on the use of computers. His main theme had been in investigating the power of a computer to rival human thought. In 1950, he published a philosophical paper including the idea of an ‘imitation game’ for comparing human and machine outputs, now called the Turing Test. This paper remains his best known work and was a key contribution to the field of Artificial Intelligence.

Why was Turing's work on AI controversial?Take an interactive tour of the Baby

We can only see a short distance ahead, but we can see plenty there that needs to be done.

Turing on the challenge of machine intelligence, 1950


A theory of life

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Why do we see patterns on animals? Prof Jim Al-Khalili and Prof Ian Stewart explain Turing’s theory. Clip from The Secret Life of Chaos (BBC Four).

Transcript (PDF 64k)

Turing turned to a completely new scientific project, which exploited his ability to use the Manchester computer.

It was the problem of understanding the biological patterns – spots, stripes, flower petals – of nature. He proposed an explanation in terms of chemical interactions and developed equations for them. His paper on this theory, completed in 1951, became a classic and is still the subject of intense investigation 60 years later. In the same year he was elected a Fellow of the Royal Society for his earlier work.

Scientists confirm Turing's theory on 'morphogenesis'


Convicted for gross indecency

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“Yours in distress.” Close friend Dr Norman Routledge reads a letter he received from Turing following his conviction. Clip from Horizon (BBC Two).

Transcript (PDF 154k)

All male homosexual activity was illegal until 1967, and Turing was prosecuted when an affair with a young man came to the notice of the police.

He made a statement lacking any element of contrition, and was treated severely. Rather than go to prison he accepted probation on the condition of having hormonal treatment which was, in effect, a chemical castration. His security clearance was revoked, ending ongoing work with the government code-breaking department – now called GCHQ (Government Communications Headquarters). His reaction was one of defiance and bravado: in particular escaping British law by going abroad to Norway and Greece.


His final year

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Did Turing take his own life or was his death an accident? Joan Murray gives her view. Clip from Horizon (BBC Two).

Transcript (PDF 108k)

Turing’s problems were not over. Defined as a security risk, he was harassed by police surveillance.

Alan Turing was found dead in bed by his cleaner on 8 June 1954. He had died from cyanide poisoning the day before. A partly eaten apple lay next to his body. The coroner’s verdict was suicide. His mother argued he had accidentally ingested cyanide during an amateur chemistry experiment, but he had probably planned his death to allow her to think this.

Colleagues share their personal memories of Alan Turing


A royal pardon

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Getty Images

The Queen visits Bletchley Park and studies an Enigma machine. She grants Turing a royal pardon on 23 December 2013.

In December 2013, Alan Turing was granted a posthumous royal pardon, formally cancelling his criminal conviction.

It followed a four-year campaign supported by tens of thousands of people, including scientists Stephen Hawking and Richard Dawkins. Opinion was divided on whether singling out an individual in this way did true justice to a situation in which thousands of gay men had been criminalised.

How was the royal pardon celebrated?

A pardon from the Queen is a fitting tribute to an exceptional man.

Chris Grayling, Justice Secretary